Can you provide a sample workbook?
You can achieve this using reference line in tableau. If you go to analytics tab there you'll get option under Custom. See the screenshot.
In your graph just drag the ref line, while putting ref line in your graph it'll give options like where you'd like to put ref line x or y axis. See the screenshot.
Put it on table, the moment you put you'll have two ref lines in your graph. By default for y-axis Tableau will generate avg line and for x-axis (which is date) min ref line. Additionally edit axis window will be appeared.
Edit the x-axis and set the line value like this
similarly edit the y-axis
Finally you'll have this view.
Let us know if this help. Workbook v10.5 attached for your reference.
Ref Lines v10.5.twbx 12.6 KB